Digital broadcasting systems, such as digital television systems, are under development, and it is intended that, eventually, they will replace analogue broadcasting systems. This is, among other things, because of their ability to provide new types of services and better quality of service capabilities compared to the analogue broadcasting systems.
One of the digital broadcasting systems currently under standardisation by the European Telecommunications Standards Institute (ETSI) is the Digital Video Broadcasting (DVB) system. In the DVB system digital video is broadcast using satellite, cable and/or terrestrial infrastructures.
The DVB system using terrestrial infrastructure is defined as the DVB-T (DVB-Terrestrial) system. In DVB-T transmission, digital data is modulated onto a radio frequency (RF) signal. The modulation method used is COFDM (Coded Orthogonal Frequency Division Multiplex) modulation. The modulated DVB-T signal is transmitted from a DVB-T transmitter. The transmission occurs over a DVB-T radio channel. The transmitted DVB-T signal is received at a DVB-T receiver. The DVB-T receiver demodulates the received DVB-T signal in order to regenerate the digital data. The digital data may contain, for example, an MPEG-2 (Moving Picture Experts Group) coded video stream.
With terrestrial digital video broadcasting it is possible to achieve a good quality data transfer even if the DVB-T receiver is mobile.
Wireless mobile communication devices are expected to be lightweight and small-sized. That is why the use of a direct conversion architecture in the DVB-T receiver is in many cases more desirable than, for example, the use of a superheterodyne architecture.
In a direct conversion receiver, a radio frequency analogue DVB-T signal, transmitted from a DVB-T transmitter and received at a DVB-T receiver, is converted directly from the radio frequency band to baseband I- and Q-signal components in the DVB-T receiver, in which “I” refers to an in-phase signal component of the DVB-T signal and “Q” refers to a quadrature-phase signal component of the DVB-T signal. In practice, the conversion is performed by splitting the received radio frequency DVB-T signal into two substantially identical signals, mixing one of the signals with a local oscillator signal to form the baseband I-signal component and mixing the other of the signals with a phase shifted local oscillator signal to form the baseband Q-signal component. The phase shifting of the local oscillator signal is performed in a phase shifter the amount of the phase shift being 90 degrees. The phase difference between the signals of an I-branch along which the I-signal component travels in the DVB-T receiver and a Q-branch along which the Q-signal component travels in the DVB-T receiver is thus exactly 90 degrees in an ideal case.
The COFDM modulation method that is used in the DVB-T system is a multicarrier modulation method. This means that, in the DVB-T transmitter, digital data to be transmitted is split into several components which are transmitted over separate carrier signals. In one of the operational modes of the DVB-T system, for example, a DVB-T channel (the bandwidth of which is 8 MHz) contains 6816 carriers (also referred to as “subcarriers”). The carriers themselves are modulated using different level QAM (Quadrature Amplitude Modulation) constellations.
FIG. 1 shows a constellation diagram showing 64-QAM constellation points. The horizontal axis, that is the I-axis, indicates the amplitude of the I-signal component of the DVB-T signal and the vertical axis, that is the Q-axis, indicates the amplitude of the Q-signal component of the DVB-T signal. In FIG. 1, the units of the I-axis and the Q-axis are arbitrary units. The I-axis and the Q-axis define an IQ-plane. Each constellation point in the IQ-plane corresponds to a transmitted bit sequence. The constellation point (I=3, Q=5), for example, corresponds to a transmitted bit sequence 001011.
It is important that the phase difference between the signals of the I-branch and the Q-branch is 90 degrees because if it differs from 90 degrees, that is there exists IQ-imbalance (more particularly IQ-phase imbalance), the probability that the transmitted bits are not detected correctly in the DVB-T receiver rises. The more complex QAM modulation is used the more sensitive the bit detection is to the IQ-imbalance.
The DVB-T system is a broadband system using a large variety of frequencies. If, for example, the UHF (UltraHigh Frequency) band is used with a channel width of 8 MHz, the used frequency band extends from 470 MHz to 862 MHz. It is difficult and expensive to manufacture a phase shifter that would perform a stable 90 degrees phase difference for the I- and Q-branches in the whole region of the used frequency band, especially taking into consideration the small size requirement of the wireless mobile communication devices.